rails polymorphic with includes based on type of class

Question

Let's say we have these models

class Message
  belongs_to :messageable, polymorphic: true
end

class Ticket
  has_many :messages, as: :messageable
  has_many :comments
end

class User
  has_many :messages, as: :messageable
  has_many :ratings
end

class Rating
  belongs_to :user
end

class Comment
  belongs_to :ticket
end

Now I want to load all messages (which have associated tickets or users), and eager load depending on the type of class, either comments for tickets and ratings for users

Of course Message.includes(:messageable).order("created_at desc") will only include the directly associated object, but the question would be how to include the different association types that derive from each model type (i.e. in this example, how to eager load comments for tickets and ratings for users)?

This is just a simple example, but what about even more complicated cases, where I'd like to include something else for the user, another association, and what if that association needs more includes?

I actually did it manually. Get the messages, and for each message type pull the associations with the corresponding includes, and then join them back in the messages array. It's not pretty, but it works — Andrei S, Mar 26 '13 at 08:57

score 2 · Answer 1 · answered Feb 26 '13 at 19:31

The only way I can think of to do this is to duplicate the associations on each model with a common name:

class Ticket
  has_many :messages, as: :messageable
  has_many :comments
  has_many :messageable_includes, class_name: "Comment"
end

class User
  has_many :messages, as: :messageable
  has_many :ratings
  has_many :messageable_includes, class_name: "Rating"
end

Message.includes(:messageable => :messageable_includes) ...

I'm not sure I would use this strategy as a widespread solution, but if this is a complicated as your case gets, it may work for you.

score 0 · Answer 2 · answered Nov 30 '13 at 12:16

I've used the following helper methods in my own project:

def polymorphic_association_includes(association, includes_association_name, includes_by_type)
  includes_by_type.each_pair do |includes_association_type, includes|
    polymorphic_association_includes_for_type(association, includes_association_name, includes_association_type, includes)
  end
end

def polymorphic_association_includes_for_type(association, includes_association_name, includes_association_type, includes)
  id_attr = "#{includes_association_name}_id"
  type_attr = "#{includes_association_name}_type"

  items = association.select {|item| item[type_attr] == includes_association_type.to_s }
  item_ids = items.map {|item| item[id_attr] }
  items_with_includes = includes_association_type.where(id: item_ids).includes(includes).index_by(&:id)

  items.each do |parent|
    parent.send("#{includes_association_name}=", items_with_includes[parent[id_attr]])
  end
end

These would allow you to say:

messages = Message.all
polymorhpic_association_includes messages, :messageable, {
  Ticket => :comments,
  User => :ratings
}

Not a particularly fluent interface but it works in general.

score 0 · Answer 3 · answered Jul 30 '14 at 10:07

Place the includes on a default scope for each model:

class Ticket
  has_many :messages, as: :messageable
  has_many :comments
  default_scope -> { includes(:comments).order('id DESC') }
end

class User
  has_many :messages, as: :messageable
  has_many :ratings
  default_scope -> { includes(:ratings).order('id DESC') }
end

Then whenever you call Message.all each polymorphic association will include it's own resources.

Also if you need to call the class without the scope just use unscoped or create a different scope:

class Ticket
  has_many :messages, as: :messageable
  has_many :comments
  has_many :watchers
  default_scope -> { includes(:comments).order('id DESC') }
  scope :watched -> {includes(:watchers)}
end

Ticket.unscoped.all # without comments or watchers (or order)
Ticket.watched.all  # includes watchers only

rails polymorphic with includes based on type of class

3 Answers3